Electrostatic conditioning electrode separator



w A. BRASTAD 3,247,960

ELECTROSTATIC CONDITIONING ELECTRODE SEPARATOR Filed June 21. 1962 WV 26 34 gi A 86} as INVENTOR WILLIAM A. BRASTAD BY/ME 4 4W ATTORNEY United States Patent Office 3,247,960 ELECTRQSTATIC CONDlTlONlNG ELECTRODE SEPARATOR William A. lllrastad, Minneapolis, Minn, assignor to General Mills, Inc, a corporation of Delaware Filed June 21, 1962, Ser. No. 204,276 11 Claims. (Cl. 209-11) The present invention relates to electrostatic separation. More particularly, the invention relates to improved apparatus and process concepts for separating conductive and non-conductive particles which are mixed together.

The development of electrostatic methods and apparatus for separations started at a relatively early date. Electrostatic methods have previously been proposed for application to ores and to mixtures of both electrically conducting and non-conducting particles.

One of these methods recognized that particles may be charged by induction. In such cases, the particles are passed into an electrostatic field, i.e., near a charged electrode, and the particles are at least temporarily grounded in some manner while still in the field. When the mixture of particles is brought into the field of an electrode and grounded, the conductors, i.e., those particles which are more conductive than the other particles of the mixture, are able to gain or lose electrons readily as compared to the non-conductors, i.e.,'those particles which are less conductive than the other particles of the mixture.

. In another method, the particles to be separated have been charged by being frictionally slid across a support made of a material selected so that one type of particle becomes charged positively and the other type negatively.

These prior methods and devices have not been totally successful for me, however, where the conductive properties of the particles to be separated were quite's'imilar, i.e., where the relative difference between the conductive properties of the particles to be separated were slight or where it is desired to obtain precise separation in short path lengths.

Accordingly, one object of the present invention is to provide an improved method and apparatus for precise electrostatic separation of conductive and non-conductive particles.

Another object of the invention is to provide an improved method and apparatus which will be capable of attaining precise and accurate separations of particles in short path lengths.

Another object of the invention is the provision of improved methods and apparatus to enhance the separation of initially charged non-conductive particles so that the conductive particles mixed therewith can be more precisely separated therefrom.

Other objects and advantages of the invention will be apparent from the following description in which certain preferred embodiments of the invention are disclosed.

According to the present invention the desired objects can be accomplished by moving a mixture of conductive and non-conductive particles in frictional engagement along a support, first subjecting said particles to an electrostatic field established between said support and a conditioning electrode with the polarity of said conditioning electrode being opposite of that acquired by the non-conductive particles by frictional electrification as they proceed along said support, and then subjecting said particles as they'proceed along said support to an electrostatic field established between said support and an attracting electrode with the polarity of said attracting electrode being opposite to that of the conditioning electrode.

By the foregoing method the frictional electrification charge of the non-conductive particles is enhanced by 3,2419% Patented Apr. 26, 1966 the conditioning electrode. That is, the charge acquired by said non-conductive particles by frictional electrification as said particles frictional slide along the support is increased by the electrostatic field established between said support and the conditioning electrode.

Since the non-conductive particles are relatively less conducting than the conductive particles, the charge acquired by the non-conductive particles as they passed through the electrostatic field established between the support and the conditioning electrode will not change as fast as the charge acquired by the conductive particles as they passedthrough the same electrostatic field when both types of particles next move through the electrostatic field established between the support and the attracting electrode. This is because of the difference in rate at which the conductive and non-conductive particles can gain and lose electrons while in contact with a grounded electrode in the presence of an electrostatic field. The time required for charging a given particle, i.e., causing it to gain or lose a certain amount of elec trons, may be likened to the time constant of a condenser. The more conductive a particle is the less time it takes to charge it, i.e., the less time it takes in a given electrostatic field to cause it to gain or lose a certain amount of electrons.

This difference in time constant between the conductive and non-conductive particles enables the polarity of the charge of the conductive particles to be rather quickly reversed by the electrostatic field established between the support and the attracting electrode while the polarity of the non-conductive particles remains the same. Thus, the enhanced charge on the non-conductive particles, i.e., the charge acquired by the non-conductive particles due to frictional electrification and the electrostatic field established between the support electrode and the conditioning electrode, provides an increased ditference between the charge of the non-conductive particles and the conductive particles as the particles move through the electrostatic field established between the support and the attracting electrode, since the charge of the con ductive particles will quickly reverse its polarity while the charge of the non-conductive particle is slow to change. This increased difference created between the charges established on the conductive and non-conductive particles as they pass through the electrostatic field of the attracting electrodes permits precise separation of particles having ditferent conductive properties in very short path lengths. Separation is accomplished preferably by providing openings in the attracting electrode so that the attracted particles, i.e., the conductive particles, will actually be projected upwardly through the openings and beyond the electrostatic field established between the support and the attracting electrode. As is known, the upper surface of the attracting electrode may then be used as a conveying surface for removal of the projected conductive particles.

It should be noted that although perforated or open attracting electrodes have been suggested in certain connections in the prior art, they have not been used in combinations with a conditioning electrode which es tablishes an electrostatic field of opposite polarity of that of the attracting electrode to condition the particles over a relatively short length of path prior to entering the electrostatic field of the attracting electrode.

The aforementioned electrostatic field may be established by means well known in the art as shown in the' drawings which form a part of this application wherein 3 FIGURE 2 is a partial sectional view on the line 22 of FIGURE 1;

FIGURE 3 is a partial sectional view on the line 33 of FIGURE 2;

FIGURE 4 is a partial perspective view showing details of the vibratory unit of the device of FIGURES 1-3.

The device of FIGURES l to 4, inclusive, is provided with a lower or support electrode designated generally as 20, a first substantially parallel upper or attracting electrode designated at 22, and a second substantially parallel upper or conditioning electrode designated at 24. These electrodes are carried by side plates 26 and 28 of insulating material. In the preferred embodiment the side plates are made of transparent plastic in order that the separating action between the supporting and attracting electrodes may be observed more readily. Attracting electrode 22 is fastened to side plates 26 by screws 30 and to side plate 28 by screws 32. Similarly, the second upper or conditioning electrode 24 is also fastened to side plates 26 and 28.

The lower electrode 2!) is connected to the side plates 26 and 28 by screws 34 and 36, respectively. These screws in turn engage wooden or insulated cross members 38 which give rigidity to the lower electrode and serve as a means of support of the assembled pair of upper electrodes and the lower electrode. These cross pieces 38 are connected to a longitudinal intermediate support 40 by bolts 42. Intermediate support 40 is provided with downwardly and forwardly inclined legs 44 and 46 which, together with the frame plate 40, constitute a substantially rigid unit.

The rear supporting leg 44 is connected by bolts 48 to an incline supporting spring 50. The lower end of this supporting spring is mounted by means of bolts 52 on a supporting shoulder 54 of a base member 56.

The forward supporting leg 46 of plate 40 is somewhat longer than the rear leg 44 as shown in FIGURE 4. A longitudinal clamping member 58 is fastened to the leg 46 by bolts 60. A horizontal or transverse spring 62 is clamped between members 46 and 58 so that leg 46 is supported at substantially the midpoint of spring 62. Spring 62 is fastened, by means of bolts 64, on supporting posts 66 projecting from an enlarged portion 68 at one end of base 55. Suitable supporting legs 79 space the base 56 the desired distance above the floor or other supporting surface.

Because of the inclination of the longitudinal and transverse springs 50 and 62, respectively, the intermediate frame 40 and the electrodes carried by it are free to vibrate resiliently in a direction substantially perpendicular to the plane of the springs. This direction is indicated in FIGURE 2 by arrow 72. Obviously the vibrations in this direction may be said to have both a vertical component, i.e., a component perpendicular to the electrodes 20, 22, and 24, and also a horizontal component, i.e., a component lying substantially in the plane of the electrodes.

In order to cause the desired high frequency vibrations to cause the particles to move along the electrodes 20 and 22, a magnet 74 is mounted on the supporting base 56 in position to attract intermittently an armature attached to the supporting leg 46. The magnet 74 is connected by wires 76 to a suitable vibratory power source 78 which includes controls 80 for adjustment of the amplitude of vibration and a connection 82 for attachment to the usual power supply line. Alternate energization and deenerization of a magnet 74 by power supply 78 will accordingly cause the desired high frequency vibrations of the frame 40 and electrodes 20, 22, and 24 in the direction of arrow '72 of FIGURE 2. These vibrations will cause particles is fed. At the lateral edges of the supporting surface 84 the electrode is provided with downwardly turned flanges 86 which are held in place between the side plates 26 and 28 and the cross members 38 by means of screws 34 and 36.

At the rear end of electrode 20 a supply hopper or chute 88 is provided. The material to be separated, indicated at 90, is prepared and fed into this hopper 88 in any desired manner. That is, it can be crushed, scalped and dried as described below, or otherwise made ready for electrostatic separation and placed in the hopper. The material is then fed from the lower end of the hopper beneath a gate member 92 which may be pivoted at 94 to the side walls of the hopper 88. The gate 92 is adjusted so the lower edge 96 is spaced slightly above the surface 84 of electrode 20, so that the material fed from the hopper 88 will move along electrode 20 in a relatively thin uniform layer.

The upper electrode 24 in this embodiment of the invention includes a relatively smooth, substantially horizontal, surface 98 under which the material to be separated is fed. The upper electrode 22 in this embodiment of the invention includes a series of openings 100 separated by intermediate flat areas 102. At the edge of each opening 100' an upwardly projecting flange or rim 104 is provided. This flange prevents material collected on the upper surface of areas 102 from passing back down through openings 100 to the lower electrode. Vertical flanges 106 at each side of the upper electrode 22 prevent inadvertent lateral removal of the material collected on the upper electrode 22.

The openings 100 are patterned over substantially the entire main area of upper electrode 22. The openings 100 are substantially five-sixteenths of an inch in diameter and are separated bya center-to-center distance of approximately five-eights to three-quarters of an inch. Near the discharge end of the device an unperforated area 108 is provided which conveys the material into a feed chute 110 from which it is deposited in a suitable receptacle or further hopper 112. This conveying action is preferably achieved by vibration of the upper electrode in a direction indicated by arrow 72 just as these vibrations also serve to frictionally convey the material in a uniform stream from hopper 88 along the upper surface of the lower electrode 20.

A further receptacle or chute 114 is provided for collection of the material discharged from the lower electrode 20. Above the discharge edge 115 of this lower electrode 20 an incline guide or shield 116 is provided. The guide 116 has side flanges 118 which are fastened to side plates 26 and 28 by screws 120'.

In order to establish the desired electrostatic field between electrodes 20 and 22, these electrodes are connected to a suitable voltage source. Thus a wire 122 is connected at 124 to the upper electrode 22. The other end of wire 122 is connected to terminal 126 of voltage supply 128. A second wire 130 is connected at one end 132 to the lower electrode 20 and is connected at its other end to the remaining terminal 134 of the voltage supply 128. The voltage supply includes a connection 136 for attachment to the usual A.C, supply line, and also includes a control member 138 for adjustment of the potential difference between terminals 126 and 134, as well as a meter 140 for indicating the potential difference for which the supply 128 is adjusted. Thus the plates 20 and 22 may be maintained at the desired potentials to establish a uniform electrostatic field between them for action on the material in question.

In order to establish the desired electrostatic field between electrodes 20 and 24 these electrodes are connected to a suitable voltage source. Thus a wire 142 is connected at 144 to the upper electrode 24. The other end of wire 142 is connected to terminal 146 of voltage supply 148. A second wire 150 is connected at one end 152 to the lower electrode 20 and is connected at its other end to the remaining terminal 154 of the voltage supply 148.- The voltage supply includes a connection 156 for attachment to the usual A.C. supply line, and also includes a control member 153 for adjustment of the' potential difference between terminals 146 and 154, as well as a meter 160 for indicating the potential difference for which the supply 148 is adjusted. Thus the plates 20 and 24 may be maintained at the desired potentials to establish a uniform electrostatic field between them for action on the material in question.

An electric heating element 162 is positioned beneath the supply hopper 88. In order to establish a current through the heating element 162, a wire 164 is connected to one end of the element 162. The other end of the wire 164 is connected to a terminal 166 of a voltage supply 168. A second wire 170 is connected at one end to the other end of the heating element 162 and is connected at its other end to a remaining terminal 172 of the voltage supply 168. The voltage supply 168 includes a connection 174- for attachment to the usual A.C. supply line, and also includes a control member 176 for adjustment of the potential difference between the terminals 166 and 172. Thus the heating element 162 may be maintained at the desired potential to obtain the desired amount of heat therefrom. One purpose of this heating element is to keep the material 90 positioned in the hopper 88 in a dry condition.

The electrostatic separation process begins with a mixture of particles some of which are conductors and others of which are non-conductors. For example, these particles can range in size between 50 mesh and 200 mesh. As used in this description, the terms conductors and non-conductors are merely relative, i.e., the particles to which the term conductors refers are more conductive than the particles to which the term non-conductors refers. Accordingly, with reference to each other, these particles are considered either conductive or non-conductive with respect to each other.

The mixture of particles is placed in the bin 83 either before or after drying. If the mixture is placed in the hopper after it is dried, it can be so dried by a number of methods well known in the art, such as by gas or oilfired cylindrical, rotary or drum type equipment. If the mixture is placed in the hopper before it is dried, it canbe dried in the hopper by operation of the heating element 162 as known in the art by properly adjusting the potential between the terminals 166 and 172. If the mixture has been dried before being placed in the hopper, it can be kept dry by operation of the heating element 162. Drying as used in this description is to be distinguished from the prolonged roasting operations sometimes practiced in the electrostatic separation art. The purpose of the drying operation is to reduce the ambient surface moisture on the particles of the mixture 90 to an inconsequential amount; for example, less than 1% by weight. That is, the particles are to be heated until, they are dry to touch and free flowing. They need not be heated so that they are bone dry. In tests conducted with mixtures of quartz, ilmenite and rutile particles, it has been found that agitating the mixture and heating at a temperature of about 230 F. is sufiicient.

From the hopper 88 the dried particles are introduced on to the lower electrode 20. The lower electrode 20 is caused to vibrate in the direction of the arrow 72 and thereby cause the particles to frictionally move along its length toward the open end 118. As is known in the art the electrode 20 is vibrated at a speed as determined by the mesh size of the particles being separated.

As the particles move along the lower electrode 20 toward the open end 118, they are subjected to frictional electrification as they move therealong and to an electrostatic field established between the lower electrode 20 and the conditioning electrode 24. The purpose of the conditioning electrode 24 is to assist the building-up ofor enhancing the charge established on the non-conductive particles by frictional electrification as said particles move along the lower electrode 20. The potential of the conditioning electrode 24 is selected as either positive or negative depending upon the polarity of the frictional electrification charge established on the non-conductive particles as they move along the lower electrode. If the frictional electrification charge is negative, then the polarity of the conditioning electrode is made positive. In the alternative, if the frictional electrification charge established on the non-conductive particles is positive, then the polarity of the conditioning electrode should be negative. The magnitude of the electrostatic field established between the lower electrode 20 and the conditioning electrode 24 is selected to be no larger than would cause the particles to be raised and to be just short of coming in contact with the conditioning electrode 24. Preferably this field would be only strong enough to cause charging of the particles, but not lifting thereof. For example, in the case of a mixture containing particles of quartz, ilmenite and rutile, the conditioning'electrode, which is positioned between /1" and vertically above the lower electrode 20, is placed at a positive potential of from 2,000 to 4,000 volts DC. with respect to the lower electrode.

As the particles subsequently move along the lower electrode 20 they are subjected to another electrostatic field established between the electrode 20 and the at tracting electrode 22. This electrostatic field is established by means of the power supply 128 as is well known in the art. The polarity of the electrode 22 is opposite to the polarity of conditioning electrode 24. The magnitude of this polarity is such that the conductive particles moving along the electrode 20 beneath the attractin'g electrode 22 will be raised sufficiently so as to pass through the openings thereof and become deposited or collected on the areas 102 of the upper portion of the attracting electrode 22. For example, in the case of a mixture containing quartz, ilmenite and rutile, the attracting electrode, which is positioned between A1" and Vs" vertically above the lower electrode, is placed at a negative potential of from 5,000 to 12,500 volts DC. with respect to the lower electrode. The non-conductive particles Will not be raised by the electrostatic field established between the attracting electrode and the lower electrode because the charge established on said non-conducting particles by the frictional electrification and subsequently enhanced by the conditioning electrode 24 is the same as that of the attracting electrode 22.

The attracting electrode 22 is caused to vibrate in the direction of the arrow 72 and thus the conductive particles are caused to move along the upper electrode 22 in a direction toward the spout 110. As is known in the art, the electrode 22 is caused to vibrate at a speed as determined by the mesh size of the particles being separated. When they reach the end of the spout they drop off into the bin 112. When the non-conductive particles reach the end of the lower electrode 20 they are discharged by the guide 116 into the receptacle 114. Thus, the conductive and non-conductive particles of a mixture can be precisely separated by the herein disclosed apparatus and method.

The theories of operation have been discussed herein in order that others may be better able to understand and apply the methods and apparatus of the invention. It should be understood, however, that the'invention is not to be limited by the particular theories advanced.

In view of the principles set forth herein, I have shown some of the ways of carrying out the present invention and some of the equivalents which are suggested by these disclosures.

What is claimed is:

1. An electrostatic separator comprising a support along which a mixture of conductive and non-conductive particles can move along a given path, means for moving said particles along said path, said support means causing said non-conductive particles to acquire by frictional electrification a charge of a predetermined polarity as said particles move along said path, a non-ionizing conditioning electrode positioned along said path for inductively charging said non-conductive particles as said particles move along said path, means for establishing a first electrostatic field between said support and said conditioning electrode with the polarity of the conditioning electrode being opposite to said predetermined polarity acquired by said non-conductive particles, an attracting electrode positioned along said path, said attracting electrode being placed so said particles first move through said first electrostatic field, means for establishing a second electrostatic field between said attracting electrode and said support with the polarity of said attracting electrode being opposite to the polarity of said conditioning electrode.

2. An electrostatic separator comprising a support along which a mixture of conductive and non-conductive particles can move along a given path, means for moving said particles along said path, said support means causing said non-conductive particles to acquire by frictional electrification a charge of a predetermined polarity as said particles move along said path, a non-ionizing conditioning electrode positioned'along said path for inductively charging said non-conductive particles, means for establishing a first electrostatic field between said support and said conditioning electrode with the polarity of the conditioning electrode being opposite to said predetermined polarity acquired by said non-conductive particles, an attracting electrode positioned along said path, said attracting electrode being placed so said particles first move through said first electrostatic field, means for establishing a second electrostatic field between said attracting electrode and said support with the polarity of said attracting electrode being opposite to the polarity of said conditioning electrode and being sufficient to cause said conductive particles to be projected upwardly from said path a predetermined distance, and means for collecting said upwardly moved conductive particles after each saidparticle has moved upwardly beyond said predetermined distance.

3. An electrostatic separator comprising a support electrode along which a mixture of conductive and nonconductive particles can frictionally slide along a given path, means for causing said particles to frictionally slide along said path, .a non-ionizing conditioning electrode positioned adjacent said support electrode for inductively charging said non-conductive. particles as said particles move along said path, means for establishing a first electrostatic field between said support electrode and said conditioning electrode with the polarity of the conditioning electrode being opposite to the polarity acquired by said non-conductive particles as said non-conductive particles frictionally slide along said support electrode, an attracting electrode positioned above said support electrode, said attracting electrode being placed so said particles first move through said first electrostatic field as said particles move along said given, path, said attracting electrode having a plurality of openings therein through whichparticles attracted from said support electrode may be projected, means for establishing a second electrostatic field between said attracting electrode and said support electrode with the polarity of said attracting electrode being opposite to the polarity of said conditioning electrode and sufiicient to cause said conductive particles to be projected upwardly through said attracting electrode openings.

4. An electrostatic separator comprising an elongated electrode along which a mixture of conductive and nonconductive particles can frictionally slide, means for causing said particles to frictionally slide along said elongated electrode, a non-ionizing conditioning electrode positioned above said elongated electrode for inductively charging said non-conductive particles as said particles slide along said elongated electrode, means for establishing and maintaining a first electrostatic field between said elongated electrode and said conditioning electrode with the polarity of the conditioning electrode being opposite to the polarity acquired by the non-conductive particles as said non-conductive particles frictionally slide along said elongated electrode, an attracting electrode positioned above said elongated electrode, said attracting electrode being placed so said particles first move through said first electrostatic field, said attracting electrode having a plurality of openings therein through which particles attracted from the elongated electrode may be projected, means for establishing and maintaining a second electrostatic field between said attracting electrode and said elongated electrode with the polarity of said attracting electrode being opposite to the polarity of said conditioning electrode and sufficient to cause said conductive particles to be projected upwardly through said attracting electrode openings.

5. An electrostatic separator comprising a substantially horizontal elongated electrode along which a mixture of conductive and non-conductive particles can fr-ictionally slide, vibratory means for causing said particles to frictionally slide along said elongated electrode, a non-ionizing conditioning electrode positioned above and substantially parallel -to said elongated electrode for inductively charging said non-conductive particles, means for establishing and maintaining an electrostatic field between said elongated electrode and said conditioning electrode with the polarity of the conditioning electrode being opposite to the polarity acquired by the non-conductive particles as said non-conductive particles frictionally slide along said elongated electrode, an attracting electrode positioned above and substantially parallel to said elongated electrode, said attracting electrode being placed so that said particles first move between said elongated electrode and said conditioning electrode as they move along said elongated electrode, said attracting electrode having a plurality of openings therein through which particles attracted from the elongated electrode may be pro ected, means for establishing and maintaining an electrostatic field between said attracting electrode and said elongated electrode with the polarity of said attracting electrode being opposite to the polarityof said conditioning electrode and sufiicient to cause said conductive particles to be projected upwardly through said attracting electrode openings. 6. An electrostatic separator as set forth in claim 5 including means for varying at least one of said electrostatic fields so the strength of said field between the conditioning electrode and the elongated electrode is less than the strength of the electrostatic field between the attracting electrode and the elongated electrode.

7. An electrostatic separator as set forth in claim 5 including means for controlling the strength of the electrostatic field between said conditioning electrode and said elongated electrode to be just strong enough to cause additional charging of the non-conductive particles with the same polarity as the charge acquired thereby due to said frictional sliding while at the same time being less than that which would cause said non-conductive particles to be lifted upwardly above said elongated electrode.

8. A method of electrostatically separating conductive and non-conductive particles from each other comprising moving said conductive and non-conductive particles in frictional engagement along a support so as to cause said non-conductive particles to acquire a charge of a predetermined polarity, subjecting said particles to a first non-ionizing electrostatic field established between said support and a conditioning electrode with the polarity of said conditioning electrode being the opposite of that acquired by the non-conductive particles by frictional electrification as they proceed along said support, then subjecting said particles as they proceed along said support to a second electrostatic field established between said support and an attracting electrode with the. polarity of said attracting electrode being opposite to that of the conditioning electrode, said second electrostatic field being of sufficient strength to cause said conducting particles to acquire a sumcient charge to move upwardly therein a predetermined distance above said support, and removing and collecting said upwardly moved particles from said field.

9. A method of electrostatically separating conductive and non-conductive panticles from each other comprising feeding said particles in a predetermined path along the upper surface of an electrically conducting electrode plate, while'maintaining said electrode plate at a predetermined electric potential, manipulating said particles as they are fed along said electrode plate by rapidly and repeatedly contacting diiierent areas of individual particles with said electrode plate and thereby causing said non-conductive particles to become frictionally electrified with a charge of predetermined polarity, subjecting said particles as they move along said path to a first non-ionizing electrostatic field established between said electrode plate and .a conditioning electrode with the polarity of said conditioning electrode being opposite to the polarity of the frictional electrification charge acquired by said non-conductive particles, subsequently subjecting said particles as they move along said path to a second electrostatic field established between said electrode plate and an attracting electrode with the polarity of said attracting electrode being opposite to that of said conditioning electrode, said second electrostatic field being of sufiicicnt strength to cause said conductive particles to acquire a sufficient charge to move upwardly therein a predetermined distance above said electrode plate, and promptly removing and collecting said upwardly moved particles from said second field after each said particle has moved upwardly beyond said predetermined distance.

10. A method of electrostatically separating conductive and non-conductive particles from each other comprising feeding said particles along a predetermined path along the upper surface of an electrically conducting electrode plate, while maintaining said electrode plate at a predetermined electric potential and maintaining first and second substantially uniform non-ionizing electrostatic fields successively along said electrode plate, manipulating said particles as they are fed along said plate by rapidly and repeatedly contacting different areas of individual particles with said electrode plate and thereby causing said non-conductive particles to become frictionally electrified with a charge of predetermined polarity, subjecting said particles as they move along said path to said first electrostatic field which is established between said electrode plate and a conditioning electrode with the polarity of said conditioning electrode being opposite to the polarity of said frictional electrification charge acquired by said non-conductive particles, subsequently subjecting said particles as they move along said path to said second electrostatic field which is established between said electric plate and an attracting electrode with the polarity of said attracting electrode being opposite to that of said conditioning electrode, said second electrostatic field being of suificient strength to cause said conducting particles to acquire a sufficient charge to move upwardly therein a predetermined distance above said electrode plate, and promptly removing and collecting said upwardly moved particles from said second field after each said particle has moved upwardly beyond said predetermined distance.

11. A method of electrostatically separating conductive and non-conductive particles from each other comprising drying said particles, feeding said dried particles in one direction along a predetermined path along the upper surface of an electrically conducting electrode plate, while maintaining the electrode plate at a predetermined electric potential, manipulating said particles by rapidly and repeatedly contacting different areas of individual particles with the electrode plate and thereby causing the non-conductive particles to become frictionally electrified with a charge of predetermined polarity, subjecting said particles as they move along said path to a first non-ionizing electrostatic field established between said electrode plate and a conditioning electrode with said conditioning electrode having a polarity which is opposite to the polarity of said frictional electrification charge, subsequently subjecting said particles as they move along said path to a second electrostatic field established between said electrode plate and an attracting electrode with the polarity of said attracting electrode being opposite to that of said conditioning electrode, said second electrostatic field being of suificient strength to cause said conductive particles to acquire a s-utficient charge to move upwardly therein a predetermined distance above said electrode plate, and promptly removing and collecting said upwardly moved particles from said second field after each said particle has moved upwardly beyond said predetermined distance.

References Cited by the Examiner UNITED STATES PATENTS 668,792 2/1901 Blake 209--128 2,187,637 1/1940 Sutton 209129 2,848,108 8/1958 Brastad 209-127 HARRY B. THORNTON, Primary Examiner.

FRANK W. LUTTER, Examiner. 

1. AN ELECTROSTATIC SEPARATOR COMPRISING A SUPPORT ALONG WHICH A MIXTURE OF CONDUCTIVE AND NON-CONDUCTIVE PARTICLES CAN MOVE ALONG A GIVEN PATH, MEANS FOR MOVING SAID PARTICLES ALONG SAID PATH, SAID SUPPORT MEANS CAUSING SAID NON-CONDUCTIVE PARTICLES TO ACQUIRE BY FRICTIONAL ELECTRIFICATION A CHARGE OF A PREDETERMINED POLARITY AS SAID PARTICLES MOVE ALONG SAID PATH, A NON-IONIZING CONDITIONING ELECTRODE POSITIONED ALONG SAID PATH FOR INDUCTIVELY CHARGING SAID NON-CONDUCTIVE PARTICLES AS SAID PARTICLES MOVE ALONG SAID PATH, MEANS FOR ESTABLISHING A FIRST ELECTROSTATIC FIELD BETWEEN SAID SUPPORT AND SAID CONDITIONING ELECTRODE WITH THE POLARITY OF THE CONDITION- 